1. Field of the Invention
The present invention relates to a laser spark plug for an internal combustion engine, the laser spark plug having a prechamber for receiving an ignitable medium and at least one overflow channel making possible a fluid connection between one internal space of the prechamber and one external space surrounding the prechamber.
2. Brief Summary of the Invention
An object of the present invention is to provide an improved laser spark plug of the type referred to at the outset for the purpose of ensuring reliable operation and longer life.
This objective is achieved in a laser spark plug of the type referred to at the outset in that the at least one overflow channel is situated and designed in such a way that when a fluid flows through the overflow channel into the internal space of the prechamber, the result is a fluid flow having at least one eddy which rotates on an eddy axis forming an angle of more than approximately 45° to a longitudinal axis of the laser spark plug.
It is particularly preferred that the at least one overflow channel is situated outside of the longitudinal axis.
According to the present invention, it has been recognized that such a fluid flow contributes favorably to preventing stagnation flows in the area of a surface of a combustion chamber window of the laser spark plug. Consequently, oil ashes or other particles arising during combustion in the prechamber accumulate to only a slight degree on the surface of the combustion chamber window, thus ensuring more reliable laser ignition and longer life than is the case in the conventional laser ignition systems having a prechamber.
In an example embodiment of the laser spark plug according to the present invention, the angle between the eddy axis and the longitudinal axis of the laser spark plug is between approximately 80° and approximately 90°, preferably approximately 90°. A fluid flow satisfying an angle condition of this type in relation to the longitudinal axis of the laser spark plug is also denoted as a so-called tumble flow.
The design of a tumble flow achieved according to the present invention makes it possible in a particularly advantageous manner to protect a surface of the combustion chamber window of the laser spark plug against the accumulation of combustion products, since it strongly reduces or eliminates a normal component of the flow velocity directed to the combustion chamber window.
According to another variant of the invention, the design of a tumble flow according to the present invention is favored in that the at least one overflow channel is situated in such a way that its longitudinal axis in the radial direction forms an angle with the longitudinal axis of the laser spark plug which is smaller than approximately 25°, preferably smaller than approximately 10°.
According to another example embodiment, it is particularly preferred that a plurality of overflow channels are provided, the longitudinal axes of adjacent overflow channels forming a maximum angle of 20°, preferably a maximum angle of 10°.
This variant of the present invention is preferred in such overflow channels, the orifices of which are situated in a face-side area of the prechamber facing the combustion chamber.
Another variant of the present invention results in a particularly intensive fluid flow according to the present invention when a plurality of overflow channels are provided which are situated and designed in such a way that the longitudinal axes of at least three overflow channels intersect at a point of intersection lying in the internal space of the prechamber. The addition of the partial flows flowing through the individual overflow channels favors the formation of a particularly strong fluid flow in the sense of a tumble flow.
A tumble flow in the prechamber may be achieved further in a particularly reliable manner in that the point of intersection lies in a radially external area of the prechamber, the point of intersection being at a distance of at least 50% to approximately 70% of a radius of the prechamber from the longitudinal axis of the laser spark plug or the prechamber.
According to another variant of the present invention, distribution of ignition flares emerging from the prechamber that is as uniform as possible as necessary for efficient ignition is ensured, if openings opening into the external space of the overflow channels, whose longitudinal axes intersect in the internal space of the prechamber, are substantially at a uniform distance from one another and/or the longitudinal axis of the laser spark plug, in particular in the area of the corners of a fictional n-corner, n being ≧3.
According to another advantageous variant of the present invention, at least one flow-guiding element may be provided which is designed for supporting the fluid flow and in particular for guiding the flow tangentially past a surface of a combustion chamber window facing the prechamber. To that end, the at least one flow-guiding element may have a flow-guiding surface, the shape of which roughly corresponds to an external contour of a tumble flow or an associated eddy which is seen as ideal.
According to another advantageous variant of the present invention, an optimal design of such a tumble flow is favored in that a combustion chamber window is provided and situated in such a way that its surface facing the prechamber is flush with one internal wall of the prechamber at least sectionally.
According to another variant of the present invention, an optimal implementation of the ignitable mixture located in the prechamber results when the laser spark plug is designed for radiating laser radiation onto an ignition point lying in the prechamber, the distance of which from a combustion chamber window is less than its distance from an end area of the prechamber facing the combustion chamber, in particular by at least approximately 50% less. This advantageously has the result that the flame core arising in the ignition point during laser ignition is carried along by the tumble flow produced according to the present invention and is guided into an end area of the prechamber facing the combustion chamber. The further development of a flame front in this area has the particular advantage that less unburned mixture is discharged from the prechamber into the external space or combustion chamber, resulting in increased efficiency of laser ignition in the prechamber.